Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature
- PDF / 892,281 Bytes
- 8 Pages / 606.24 x 786 pts Page_size
- 33 Downloads / 217 Views
31/7/03
9:01 PM
Page 1985
Nanostructure in an Al-Mg-Sc Alloy Processed by Low-Energy Ball Milling at Cryogenic Temperature F. ZHOU, S.R. NUTT, C.C. BAMPTON, and E.J. LAVERNIA Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of 26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and 24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.
I. INTRODUCTION
MECHANICAL attrition (MA), or ball milling, has been widely used to synthesize nanostructured materials,[1,2] normally defined as materials with a grain size of less than 100 nm. This method induces severe cyclic deformation in powders and can be used to synthesize a variety of materials. The MA process produces nanostructures by the structural decomposition of coarser-grained structures as the result of severe plastic deformation, although the exact mechanism is not well understood. The MA process is generally performed at room temperature and in certain instances at liquid-nitrogen temperature. In both cases, the process is considered as cold deformation. The development of nanostructures is reportedly dominated by lattice strain,[1] regardless of the material system and processing variables, such as mill energy and milling temperature. Recently, ball milling at cryogenic temperatures (cryomilling) has been successfully used to synthesize several different nanostructured metals and alloys,[3–11] including ductile materials such as Al and its alloys. In general, some difficulties arise during ball milling of Al-based alloys when conventional ball milling techniques are employed because of the relatively strong tendency of powder particles to adhere to container walls, to agglomerate, and to sinter to form large millimeter-sized particles. The introduction of cryogenic liquid media during milling effectively eliminates this
F. ZHOU, Assistant Specialist, formerly with the Department of Chemical Engineering and Materials Science, University of California, Irvine, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294. Contact e
Data Loading...
